The present invention relates to a lubricant particularly suitable for lubricating internal combustion engines.
For many years, zinc dialkyldithiophosphates (ZDPs) have been used in engine lubricants to provide antiwear and antioxidant performance. Despite their excellent history of performance benefits, these materials do contribute sulfur and phosphorus content to lubricating oils. Recently environmental and other requirements have placed increasingly stringent limits on the amount of sulfur and phosphorus that can be contained in engine lubricants. One possible response to this demand is to reduce the amount of ZDP and other sulfur- or phosphorus-containing additives such as sulfurized olefins and sulfur-containing detergents in the lubricants. Not unexpectedly, this approach has led to worsening of exactly those properties that ZDP and other such materials were originally added to improve. Performance areas which are particularly severely affected include antiwear performance of the injector adjustor screw, as measured by the “Short Cummins M11 Engine Test,” as well as performance in the Cummins M11 Engine Test and the Cummins M11 Test.
To improve the antiwear performance of such formulations while maintaining acceptably low sulfur and phosphorus levels has been a significant technical challenge. Although small amounts of sulfur- or phosphorus-containing compounds may be added if they impart significant benefits, the amount employed must not be so great as to raise the amounts of S or P to unacceptable levels. It is also desirable that the lubricant formulation will be low metal-containing ash (less than 1.2% or 1% sulfated ash by ASTM D 874, preferably less than 0.8%, 0.6%, or 0.5%), so preferably any additive will contribute little or no sulfated ash. (If desired, the lubricant may also have a small level of sulfated ash, e.g., 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%.)
Various approaches to lubricating internal combustion engines are known. U.S. Pat. No. 4,582,618, Davis, Dec. 14, 1984, discloses low phos- phorus- and sulfur-containing lubricating oils, containing less than about 0.1% wt. phosphorus, more generally less than about 0.08%. A sulfurized Diels-Alder adduct can also be present. Such compositions are said to exhibit improved antiwear properties.
U.S. Pat. No. 5,006,271 Avery et al., Apr. 9, 1991, discloses sulfurized propylene based lube olefin derivatives. Fully formulated lubricating oils may contain a variety of other additives. Examples include 1% of a sulfurized propylene based lube olefin in a standard test mineral oil.
U.S. Pat. No. 5,719,107, Outten et al., Feb. 17, 1998, discloses crankcase lubricants for heavy duty diesel, containing at least 4 mass % dispersant, at least 0.3 mass % metal phenate, less than 0.1 mass % friction modifier, less than 0.3 mass % sulfurized phenols, and less than 0.12 mass % neutral calcium sulfonate. Oxidation inhibitors include sulfurized hydrocarbons.
U.S. Pat. No. 2,227,692, Barnard, Jan. 7, 1941, discloses oils for lubricating diesel engines, which includes a small amount of sulfur or a sulfurized mineral oil.
The present invention solves the problem of providing a low sulfur, low phosphorus, low ash lubricant which exhibits good antiwear performance, by including in the formulation a minor amount of a sulfurized olefin.